When I Called Into The Shop A Couple Of Days Ago,

Graham gave me one of these, back in the pre 1970s, a essential piece of kit for aquarium hobbyists, 

but what is it? (I have blanked out the box with its description), a few clues, it is to be connected into the mains electricity supply, 220 -  240 volts, but the unit does not use any electricity, one wire in and one wire out, both mains voltage, have a few moments to try to figure it out, 

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it is a bi-metal strip thermostat, known as a ET5, with a security cover to stop inadvertent turning of the temperature control,  

the unit has quite a pedigree, all that happens is that the steel clip holds the unit to the outside of your aquarium, the bi-metal strip inside the unit has a contact at one end, and a screw at the other end, the dial adjust the tension of the strip via the screw, when contact is made electricity passes through the unit to a heating element in a glass tube which is in the aquarium,

and here it is illustrated, I must have explained how to wire these up so many times before the introduction of a heater and thermostat in one submersible tube, I should mention in the day aquarist that had a fish room/shed could run a number of aquariums off of the same unit, if they were at the same level, and same size, one aquarium with the thermostat and several others with just the same size heater in each one, a part of the aquatic trade that has long since gone, seeing it brought back so many memories of when I was a Saturday boy at the shop in the early 1960s, and then subsequently worked there full time.